There's an archived webchat on the First Capital Connect site that provides some concrete information of what will happen if the Thameslink Programme gets funding (announcement due very soon). There's also some info on it in the Network Rail Business Plan. Cross-referencing with existing info, it looks like the plan is:
- Key Output 0: By December 2008, the central section will have been upgraded to allow 15 trains per hour (tph). The Moorgate branch and the Blackfriars bay platforms will then be closed to allow the main construction to happen, and the displaced services will have to run all the way through, with Thameslink trains from the north going on to serve some of Southeastern's current routes.
- Key Output 1: By December 2011, all (or most) stations will have been rebuilt for 12-car trains. This would have to include the full rebuilds of Blackfriars and Farringdon.
- Key Output 2: By December 2015, 24 tph will be running through the core section. This requires rebuilding London Bridge station, and the remaining major works.